Asset Retirement Obligations (AROs) (Reconciliation Of Changes In Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Asset Retirement Obligations [Line Items]
Balance, Jan.1	$ 91.1		$ 75.9
Revisions in estimated cash flows	(8.2)	[1]	(0.3)	[1]
Liabilities settled	(1.0)		(0.1)
Accretion expense	0.9		1.1
Balance, March 31	82.8		76.6
IPL [Member]
Asset Retirement Obligations [Line Items]
Balance, Jan.1	56.2		43.6
Revisions in estimated cash flows	(8.2)	[1]
Liabilities settled	(0.9)		(0.1)
Accretion expense	0.5		0.7
Balance, March 31	47.6		44.2
WPL [Member]
Asset Retirement Obligations [Line Items]
Balance, Jan.1	34.9		32.3
Revisions in estimated cash flows			(0.3)	[1]
Liabilities settled	(0.1)
Accretion expense	0.4		0.4
Balance, March 31	35.2		32.4
Sixth Street [Member] | IPL [Member]
Asset Retirement Obligations [Line Items]
Revisions in estimated cash flows	$ (8.2)

[1] In the first quarter of 2012, IPL recorded revisions in estimated cash flows of $8.2 million based on revised remediation timing and cost information for asbestos remediation at its Sixth Street Generating Station.